SIVstm was isolated from a rhesus macaque which developed AIDS-like disease upon inoculation with lymph node homogenate of a stump-tailed macaque which had died from a lymphoma. A 2.0 kb fragment containing the 5' LTR and gag region of SIVstm was amplified by PCR from virus-infected human PBL DNA. Nucleotide sequence comparison of the gag regions of SIVstm and other SIVs indicated that SIVstm had 84% homology to SIVs isolated from Asian pig-tailed (SIVmne) and cynomolgous (SIVmac) macaques and African sooty mangabey (SIVsmm). SIVmne and SlVmac (which are 96% homologous to each other and 87% to SIVsmm) are thought to have originated in captivity by cross-infection of Asian macaques by SIV of sooty mangabey. The nucleotide sequence divergence of SIVstm from the other SIVs suggests that stump-tailed macaques were infected a virus different from that of the sooty mangabey.